position rack ion lower third of oven.
Heat oven to 400.
Season chicken with 1 tbsp chopped rosemary, 1/2 tsp salt, and 1/4 tsp pepper.
In a bowl, toss squash with 1 tsp oil. In a second bowl, toss onions with 2 tsp vinegar and 1 tsp oil.
Coat a rimmed sheet pan with cooking spray.
Place chicken on pan. spread veggies around chicken in a single layer.
Sprinkle veggies with remaining rosemary, salt and pepper.